The present invention is directed to echo cancelers which provide echo cancellation on communication channels carrying digitized signals. The invention is more specifically directed to a stand-alone echo canceler capable of automatically enabling or disabling echo cancellation on a per-call basis.
In telecommunication systems it is common to utilize separate transmit and receive transmission channels to carry communications between subscribers. An interface between the separated transmit and receive channels and a two-way communication channel coupled to normal subscribers is typically accomplished by a 4-wire to 2-wire hybrid. Impedance mismatches terminating the transmission lines at the hybrid result in incomplete cancellation of an incoming signal, e.g. the other party's speech, causing an attenuated replica of the incoming signal, an echo, to be carried on the outbound channel. If the echo is not attenuated, it may provide an annoying signal to the subscriber which originated the speech. Echo cancelers typically located near the 4-wire to 2-wire conversion location process the inbound communication signals and generate an echo cancellation signal which is 180 degrees out of phase with the echo signal. The echo cancellation signal is added to the outbound channel in time synchronization so as to effectively cancel the echo signal. Echo cancelers are available to process both analog and digitally encoded signals carried on the 4-wire transmission channels.
It is normally desirable to provide echo cancellation where conventional voice signals are being transmitted in order to eliminate or minimize an undesired return echo. However, there are circumstances in which the utilization of an echo canceler for a given call is undesired. For example, a modem which uses tone signaling to convey information operating in a full duplex mode may be impaired by the echo canceling signal injected on the return path. Other types of digital data transmission, especially when operating in a full duplex transmission mode, may also be adversely impaired by the introduction of a signal produced by an echo canceler intended to cancel the return echo. Therefore, it is desirable to be able to control whether or not an echo canceler is utilized.
Various types of techniques have been developed for controlling echo cancelers. Typically, modems operating in accordance with CCITT/ITU-T standards may transmit a predetermined tone prior to the initiation of communications on a channel as a signal to disable echo canceling for the call. Proprietary control signals generated by central office switches are sometimes transmitted to associated echo cancelers in order to provide control of the echo canceler. Such proprietary systems may require communication paths separate from the traffic carrying channel between the equipment generating the control signals and the echo cancelers in order to communicate the control information to the echo canceter. Thus, a central office or toll switch can control whether an associated echo canceler will be active or inactive. Such echo canceling techniques have generally proved successful for many applications.
However, such echo canceler use has proved unsatisfactory due to unreliability, lack of stability, or improper function under some data transmission requirements. For example, a private ISDN network may require echo cancellation on voice calls while also requiring that echo cancellation be disabled on calls in which data is being transmitted. One solution is to separate and identify the channels which carry voice and data calls. Echo cancelers can be placed only on the channels which carry the voice calls. While this technique provides a solution, it limits the users of the network to fixed choices between voice and data channels. Another possible solution is to use control signals for echo canceler control generated by a private branch exchange (PBX) or switching equipment in the network coupled to the PBX. However, such control signals may be incompatible since different proprietary control signals may be encountered. Where separate channels are needed to transmit the control signals to an echo canceler, separate channels may not be available or may be available only at substantial cost. Thus, a need exists for an improved echo cancellation technique which accommodates voice or data calls to be made on the same communication channel.